The present invention relates to a method of controlling the combustion process in a combustion chamber in an internal combustion engine with at least one cylinder having at least one inlet valve and one exhaust valve, means for varying the geometric compression ratio of the cylinder and means for supplying a homogeneous fuel/air mixture to the combustion chamber.
The invention also relates to a four-stroke internal combustion engine with at least one cylinder having at least one inlet valve and one exhaust valve, means for varying the geometric compression ratio of the cylinder as well as means for supplying a homogeneous fuel/air mixture to the combustion chamber of the cylinder.
For homogeneous charge compression ignition (HCCI) in a four-stroke internal combustion engine, a homogeneous diluted (with extra air or residual gas) fuel/air mixture is compressed to self-ignition. The advantage of this compared to first compressing the inlet air and then injecting fuel into the combustion chamber (the diesel process) is that the entire fuel/air mixture bums simultaneously and not successively as when a flame front propagates through the combustion chamber from a sparkplug or injector. This creates a homogeneous temperature in the combustion chamber, which in turn makes it possible to achieve, for example in an unthrottled Otto-engine at partial load, the efficiency of the diesel engine but without the high nitrogen oxide and particle emissions of the diesel engine. The nitrogen emissions can be reduced from ca. 1000 ppm to as little as 10-20 ppm. The particle emissions of the diesel engine can be reduced to the same level as those of the Otto-engine. The difficulty is, however, to control the combustion since it is kinetically controlled. If the mixture is too rich, the energy released will be too rapid (knocking), and if it is too lean, ignition will be made impossible. In an HCCI Otto-engine with gasoline as fuel, a high and controlled temperature is required to achieve self-ignition, and this can be achieved with high compression ratio and/or by heating the inlet air. In an HCCI diesel engine with diesel oil as fuel, lower temperatures are required than with a normal diesel engine, which means that the compression ratio must be lowered.
The difficulty up to now in HCCI-engines has been to control the ignition delay (the cylinder temperature) in such a manner that the combustion is positioned correctly about the top dead centre at various rpm:s and loads, and this has greatly reduced the range of use of such engines. Especially control problems during transients, where the cylinder temperature must be checked from one cycle to the next, has limited the range of use of HCCI-engines to generators, for example, where the drive unit operates with very small variations in rpm and load.
The purpose of the present invention is to achieve a method of controlling the temperature in the cylinders in an HCCI-engine, so that the ignition time will be correct at various engine speeds and loads, thereby making it practically possible to use HCCI-engines in motor vehicles, thereby reducing their fuel consumption and emissions.
This is achieved according to the invention by virtue of the fact that the compression ratio and the closing of the inlet valve are controlled so that the mixture, at least within a lower rpm range, is compressed to self-ignition.
Complete freedom of valve control, so that the opening and closing time can also be freely controlled from cycle to cycle, can be achieved by using electromagnetically operated valves. The compression ratio can be varied in a known manner by virtue of the fact that the engine cylinder communicates with an additional cylinder containing a movable plunger by means of which the total volume of the combustion chamber can be varied.
An HCCI-engine which must be able to operate within a wide rpm range, e.g. with an upper rpm limit of about 6000 rpm, is preferably equipped with an ignition system which is controlled so that it is deactivated within said lower rpm range, the upper limit of which can lie between 3000 and 4000 rpm. When this limit is exceeded, the ignition system is activated at the same time as the control of the inlet valve is changed and the compression ratio is reduced to normal engine operation.
An internal combustion engine of the type described by way of introduction, which is to be controlled in the manner described above, is characterized in that at least the inlet valve has variable valve times and that the control means are arranged to control the degree of opening of the inlet valve and the compression ratio of the cylinder as a function of engine speed and load, so that the mixture, at least within a lower engine speed range, is compressed to self-ignition.